In a conventional image forming device which performs printing or processing operations on sheets, such sheets are accommodated in a sheet accommodation tray in a stacked state and are picked up one sheet at a time when supplied. One of conventional sheet supplying units includes a pivotally movable arm portion and a pick-up roller provided at a free end of the arm portion. The pick-up roller contacts sheets stacked in a sheet accommodation tray and picks up each sheet while rotating. In this sheet supplying unit, when the sheets within the sheet accommodation tray become fewer, the pick-up roller is configured to pivotally move downward to pick up each sheet.
However, in this sheet supplying unit, heights of uppermost sheets in the sheet accommodation tray change depending on how many sheets are left therein. Therefore, a time period until when a leading edge of each sheet reaches a prescribed position in a sheet conveying path from when each sheet is picked up also varies in accordance with amounts of sheets left in the sheet accommodation tray. That is, conveyance distance is practically longer when fewer sheets are left, thus requiring longer time. Note that, the prescribed position in the sheet conveying path is assumed to be such a position that a detection sensor for detecting passage of sheets is provided. When a plurality of sheets is sequentially conveyed, since a predetermined time interval is necessary to be interposed between two consecutive sheets, the sheet supplying unit is required to start feeding subsequent sheet when the detection sensor detects a trailing edge of a precedent sheet. Therefore, when a trailing edge of a sheet is detected to have passed, a prescribed period of stand-by time has been conventionally established before a new sheet is supplied.
Since the conveyance distance up to the detection sensor varies according to amounts of sheets in the sheet accommodation tray, time intervals between each sheet inevitably change when a plurality of sheets needs to be supplied consecutively. Thus, how many sheets are supplied in a certain time period consequently varies. As a result, sheet feeding capability cannot be fully utilized in such a conventional sheet supplying unit.
Another type of conventional sheet supplying units has proposed a movable pressure plate provided on a bottom surface of a sheet accommodation tray. The pressure plate is configured to be movable in a vertical direction so that sheets stacked in the sheet accommodation tray are urged upward and a pick-up roller can therefore pick up an uppermost sheet at a constant position.
This sheet supplying unit appears to obviate the drawbacks described earlier. However, the sheets below an uppermost sheet tend to be slightly displaced when the pick-up roller picks up the uppermost sheet Hence, positions of uppermost sheets thus slightly vary time to time within the sheet accommodation tray. As a consequence, considering such positional variations of sheets, a longer time interval is necessary to be set in order to secure a predetermined time interval between consecutively supplied sheets at the time of sheet supply.